At 3 p.m. today, 16 vessels will converge off Newport, R.I., and embark on the bark of a gun on this earth's longest odyssey: a voyage around the world, powered by wind alone, one man per boat.

"It was just something that had to be done," said bearded David White, one of the 16 skippers and the man who dreamed up the notion of a round-the-world, singlehanded sailboat race and saw it to fruition. "We figured why not now, and why not start in the United States? If we hadn't done it, the French or the English would."

White will pilot a red 56-foot sloop he renamed Gladiator after race sponsors disallowed his first name, Death Wish, on grounds it would reflect badly on the spirit of the race.

For those who think the first version makes more sense, we turn to Robin Knox-Johnston, who won history's only other round-the-world solo race in 1969 when he sailed 30,123 miles in 313 days. Nine boats entered that race; only Johnston's finished.

Johnston, chairman of this race, thinks the ratio of finishers will be higher this time--probably about 50 percent. He surveyed the yachts at Goat Island Marina's B Dock the other day and said, "All these boats are capable of completing the course. It's the skippers who will determine which actually do."

The skippers are a curious mix. There is Yukoh Tada, 52, a Tokyo cab driver and Zen believer who will carry an electric piano, a tenor sax and practically no other amenities aboard his 43 1/2-footer; Tony Lush ("The only genuine Lush in the race," by his account), who slapped together the hull of his 54-footer with friends in five days; Tommy Lindholm, 57, a former policeman and former mayor of Hidden Hills, Calif., piloting the family cruiser, and Paul (Jolly) Rodgers, a mournful Briton who evidently didn't get enough punishment the first time when he sailed solo 1 1/4 times around the world and who regards this as "the most dangerous sailboat race ever."

He may be right.

The course is in four legs, Newport to Cape Town, South Africa; Cape Town to Sydney, Australia; Sydney to Rio de Janeiro and Rio back to Newport, presumably by sometime early next summer.

The two middle legs involve rounding the feared capes of Good Hope and Horn and sailing through the "Roaring 40s," where temperatures dip below freezing and winds blow wildly and unpredictably. That should be tough sailing.

"Not the worst," said Rodgers. "The worst danger is that they're starting this thing right in the height of hurricane season, with a course directly in the middle of hurricane territory." There isn't a yacht in the race, said Rodgers, that could be expected to survive 100-mile-per-hour hurricane winds with only one man to keep her going.

With that and other factors in mind, most of the racers seem more concerned with finishing than winning, despite $25,000 prizes for first place in the small-boat (44 feet and under) and big-boat (45 to 56 feet) classes. Said Tada, "Competitor is the wrong word. It should be 'participant.' "

And indeed, just trying to finish makes sense. If Johnston's guess is right and only half the starters make it, the eight who managed to return to Newport intact would have a 25 percent shot at the two prizes.

The $50,000 and other organizational costs were covered by the sponsoring BOC Group, an international manufacturer of industrial gases. The sponsorship didn't come until last winter and, as a result, the field is small and not as competitive as it might have been.

But sailing buffs familiar with the sudden popularity of singlehanding, as evidenced by the crowded field for recent OSTAR transatlantic races, consider this a trial. "If this race goes well," said Lush, a two-time OSTAR finisher, "you'll see a lot of hotshots in the next one (in 1986)."

In the melange of not-so-fast-looking racers at B Dock this week, a few speedsters stood out. Credit-Agricole is a 56-footer built specially for the race and sponsored by a bank to the tune of $116,000. Frenchman Philippe Jeantot, a professional deep-sea diver, will pilot her.

South African Bertie Reed arrived, fresh from breaking the record for sailing the fastest monohull in the Round-Britain race, in his 49-foot Altech Voortrekker. It is well equipped and race-ready.

Among those deemed likely to complete the course is Francis Stokes of Annapolis, whose 39-footer Mooneshine was so well-prepared that he took off last week to clean up business and personal details at home while his yacht awaited him at the dock.

What makes a fellow give up a year of his life for the uncertainty and hardships of a solo voyage around the globe? "Just putting the 'x' on the chart every day," said the Englishman, Rodgers. "That's really the only satisfaction I need."